Acoustic-based distance measuring systems have been used to compute the position of a data entry object in a writing field for some time. For example, schemes that track and record the position of a pen on a white board are commercially available. As the user writes on the white board, the transcription system determines the location of the pen on the board and records the location for later use.
In such systems, a conventional marking pen of the type used with white boards is inserted into a housing that includes an acoustical transmitter and an infrared transmitter. As the user writes on the white board in the conventional manner, the transmitter sends a combination of acoustical and infrared pulses. Two receivers that are separated in space receive the signals generated by the housing. Each receiver measures the time difference between the time of arrival of the infrared pulse and the acoustical pulse to determine the distance of the housing from that receiver. These distance measurements are then combined to determine the position of the housing relative to the receivers.
Infrared is used for the light signals to avoid problems with background light in the area of use. The acoustical signals are typically in the ultrasound range so that the signals are beyond the human audible range. In addition, the higher frequencies provide better spatial resolution. Each acoustical receiver is typically constructed from a microphone such as ceramic piezo microphones, PVDF films, condenser microphones, electrets condenser microphones (ECMs), moving coil microphones, etc.
Unfortunately, the sensitivity of these devices as utilized in prior art systems is not completely omni-directional at ultrasound frequencies. The variation in angle with respect to each sensor over the range of positions of the pen on the surface can be relatively large. Hence, angular variation in the gain of the ultrasound receivers can lead to increased errors due to noise and variation in the trigger point on the ultrasound pulse as a function of angle. The latter type of error results in an error in the perceived delay time of the ultrasound signal, and hence, an error in the calculated distance from the sensor to the pen. In the extreme case, the microphone can have insufficient gain to detect the pen in some regions of a large writing surface. These gain problems can limit the size of the work surface that can be transcribed.
In addition to an omni-directional detection profile, the frequency response of the sensor is also important. Even at ultrasound frequencies, there are narrow band background ultrasound sources that can interfere with the reception of the ultrasound pulse from the pen. For example, some motion detectors utilize an ultrasound signal to detect an object moving within the field of view of the motion detector. These narrow band sources can have a signal strength that is sufficient to mask the ultrasound signal from the pen in the transcription system. The transcription system pen is typically battery powered, and hence, cannot compete with a motion detector that is powered from an AC power source and generates a signal having an amplitude that is sufficient to detect the change in frequency of the signal after the signal has been reflected from a moving object.